This invention relates generally to pneumatically-operated process control systems, and more particularly to an improved pneumatic relay for such systems.
The typical industrial process control system is operated by one or more final control elements, such as valves, adapted to govern the supply of fluid to the process. Where the final control element is a pneumatically-operated diaphragm type valve, it is known to actuate this valve by means of a pneumatic transmitter that is manually settable by an operator. This transmitter functions to transmit air to the final control element at a pressure level within a desired process control range, such as 3 to 15 PSIG. A manual transmitter of this type generally includes a pneumatic relay to increase the air handling capacity of the system.
In a conventional pneumatic relay for this purpose, two diaphragms are used, one of which covers an output chamber into which air from a constant pressure supply is fed through an adjustable supply port. The other diaphragm covers the input chamber into which is fed an input air signal representing the variable to be transmitted.
In a manual transmitter, this input signal is derived from an external fixed-volume chamber into which pressurized air is introduced under manual control so that the fixed volume chamber supplies an air input signal to the relay representing the variable to be transmitted--say, a signal having a pressure of 9 PSIG. If one wishes to change this air input signal to, say 5 PSIG, the fixed volume chamber is vented to the atmosphere until the internal pressure thereof is reduced to this level. In the relay, an exhaust chamber is defined between the two diaphragms. This exhaust chamber contains atmospheric air and a spacer to transmit the force between the output and input chambers, the spacer containing a relay exhaust port.
The deflection of the diaphragms, in response to air pressure in the input chamber, acts to open the supply port to feed air from the constant pressure supply into the output chamber until the pressure therein is in equilibrium with the pressure in the input chamber, at which point the pressure of the air output signal yielded in the output port is proportional to the input signal. A subsequent change in the air input signal resulting in a corresponding change in pressure in the input chamber produces a pressure differential across the diaphragms between the input and output change to cause the relay exhaust port to open to reduce the pressure in the output chamber until equilibrium is restored.
The practical difficulty with a conventional double diaphragm relay of the above-described type is that the diaphragm is not completely impermeable to the flow of air therethrough; for existing diaphragm materials suitable for pneumatic relays, such as neoprene, possess some degree of permeability to air, the extent of molecular air flow through the diaphragm wall depending on the pressure difference thereacross. This air leakage through the diaphragm results in a reduction of pressure within the input chamber which is supplied by a fixed volume external to the relay, thereby impairing the maintenance of the desired output air signal.